Solder masks are patterned films or layers which are designed to permanently overlie the printed circuitry on a printed circuit board. Patterned openings in solder masks selectively permit solder to adhere to metal of the printed board. The solder mask also protects the circuitry against short-circuiting.
Because solder masks are designed to provide a permanent layer, hardness and durability of the layer are considered important features of a solder mask composition. Among the hardest and most durable of photomask compositions are those which are epoxy-based. Such compositions generally comprise an epoxy resin dissolved in a solvent. A major disadvantage of epoxy compositions is that they are imagewise screen printed, a primitive technology compared to much more desirable photoimaging.
Photoimageable compositions which may be used to form solder masks or the like typically contain a film-forming material, a photo-polymerizable monomer, e.g., an acrylate monomer, and a photoinitiator. Examples of such compositions are described in U.S. Pat. No. 4,528,261. Although such compositions have the advantage of being photoimageable, they are not as strong or as durable as those based upon epoxy resins.
U.S. Pat. No. 4,268,614, the teachings of which are incorporated herein by reference, describes photoimageable compositions which include acrylic resins, acrylic monomers and epoxy resins. The photoimageable compositions described in this '614 patent may be applied as a solution in an organic solvent directly to a circuit board or applied as a solution to a base sheet so as to form a dry film. Compositions such as described in the '614 patent which form a film upon evaporation of an organic solvent are disadvantageous with respect to the use of high levels of organic solvents. In forming films from solutions, the organic solvent typically comprises about one-half of the solution volume. The cost of the solvent, including the shipping and storage of the same, is quite significant. Furthermore, organic solvents, which must be evaporated from the layered compositions, pose substantial health and environmental problems. A further disadvantage of the photoimageable compositions described in the '614 patent is that once photoimaged, the film is developed by washing away the unexposed areas with an organic solvent.
It is an object of the present invention to provide a photoimageable substance which at or near 100% solids may be formed into a film, exposed, developed with aqueous solution and thereafter cured to form a layer with hardness and permanency characteristics similar to films based entirely upon epoxy resins.